1. Field of the Invention
With the forthcoming entry into force of the Euro IV standards on exhaust emissions for heavy goods vehicles, devices for removing NOx (or oxides of nitrogen) pollution need to be fitted.
2. Description of the Related Art
The system adopted by most heavy goods vehicles manufacturers to reduce NOx emissions to the required value generally consists in performing selective catalytic reaction using reducing agents such as urea (urea “SCR” for selective catalytic reduction, using the ammonia generated in situ in the exhaust gases by the decomposition of the urea).
To do this, it is necessary to equip the vehicles with a tank containing a solution of urea, and with a device for metering the amount of urea to be injected into the exhaust line. Given that the aqueous urea solution generally used for these purposes (with an eutectic at 32.5% by weight of urea) freezes at −11° C., it is necessary to provide a heating device in order to liquefy the solution so that it can be injected into the exhaust line under cold-start conditions at freezing temperatures.
Several systems have been provided for this purpose in the prior art. In general, these systems comprise fairly expensive heating devices involving either special-purpose heating elements or a bypass on the engine cooling circuit.
US application 2003/0101715 proposes to solve this problem by using the fuel return line present on certain engines (direct injection diesel engines, particularly of the “common rail” type) to heat the additive. However, when the outside temperature is high enough, there is no need, and it is even sometimes not desirable, to heat the urea solution because doing so might cause its premature decomposition and the emission of undesired odorous vapours (which is something that already occurs upwards of 60° C. with the aforementioned eutectic point). Furthermore, the system proposed in that document is unable effectively to cool the diesel return in the event of high temperatures.
This application aims to solve these problems by proposing a method and a system for storing a reducing additive and injecting it into the exhaust gases of an engine which is effective and not excessively odorous irrespective of the ambient temperature and which does allow effective cooling of the fuel return if necessary.